Nowadays, the 3rd Generation Partnership Project (generally known as “3GPP”) is developing a so-called IP Multimedia subsystem (generally known as “IMS”), which is described in the 3GPP document “Technical Specification Group Services and System Aspects; IP Multimedia (IM) Subsystem (3GPP TS 23.228 version 6.8.0)” (hereinafter referred to as 3GPP TS 23.228). This IP Multimedia subsystem (IMS) comprises elements for provision of multimedia services, such as voice, video, messaging, data and web-based technologies, as well as communications with legacy telephony systems. In a wide context, the IP Multimedia subsystem (IMS) thus includes a collection of signalling and bearer related network elements for providing its users with IP multimedia services. These IP multimedia services are based on an IETF defined session control capability that, along with multimedia bearers, utilises the IP-Connectivity. In this context, the multimedia session is the signalling corresponding to multimedia bearers.
Generally speaking, a user accesses to the IP Multimedia subsystem through an access network where the user has previously attached himself and from where the user gets IP connectivity. In particular, the IP Multimedia subsystem may have infrastructure in a Public Land Mobile Network (PLMN) where the user is a subscriber, namely in the Home PLMN (HPLMN) or home network, or in a Visited PLMN (VPLMN) or visited network where the user is roaming.
The IP Multimedia subsystem (IMS) is aimed to be an access-independent domain under the umbrella of the 3GPP standardization bodies, and may be thus accessible through any access network operating in accordance with a General Packet Radio System (GPRS) network, or a Wireless Local Area Network (WLAN) hotspot, or a Broadband network, or a Code Division Multiple Access (CDMA) network such as a so-called CDMA2000 or a Wideband CDMA (WCDMA), or even a wired-line network, the latter with minimal adaptations.
Once the user has attached to a selected access network, the user submits to the access network, implicitly or explicitly, an indication to further register into the IP Multimedia subsystem in order to get IP connectivity and to be assigned an entry node to said IP Multimedia subsystem by means of appropriate protocol mechanisms available at the selected access network. In accordance with the above 3GPP TS 23.228, this entry node to the IP Multimedia subsystem is a so-called Proxy Call Session Control Function (P-CSCF) that behaves like a Proxy by accepting requests and servicing them internally, or forwarding them towards another entity like an Application Server.
The registration of a user into the IMS through a selected access network is nowadays carried out with a so-called Session Initiation Protocol (hereinafter SIP) Register message, originated from the given user towards the P-CSCF. This SIP Register message is forwarded from the P-CSCF towards a so-called Interrogating Call Session Control Function (I-CSCF) included in the IMS infrastructure, which is in charge of selecting a Serving Call Session Control Function (S-CSCF) for serving the user by performing, among others tasks, service control.
After having selected an S-CSCF, the I-CSCF forwards the user-originated Register message to the selected S-CSCF that, upon reception of this message, requests an IMS Service Profile for the given user from a so-called Home Subscriber Server (HSS) that holds subscriber data for the user. The IMS Service Profile, which is a sort of user profile, is a collection of service and user related data as defined in the 3GPP document “Technical Specification Group Core Network; IP Multimedia (IM) Subsystem Cx and Dx interfaces; Signalling flows and message contents (3GPP TS 29.228 version 6.5.0)” (hereinafter referred to as 3GPP TS 29.228).
Once the applicable IMS Service Profile for the user is received at the S-CSCF from the HSS, the S-CSCF may from now on perform session control services for the user, including services depending on the type of call, and maintain a session state as needed by the network operator for support of services.
Two significant aspects of the IMS development contribute to somewhat limit those services for which dialled number internationalization may apply and, even more, when applying services that depend on the type of call, national, international, or in selected countries. In this respect, the handling of dialled number formats for the purpose of the present invention is described in 3GPP TS 23.228 6.8.0. A first aspect is the selection of an S-CSCF in the home network for servicing the user, so that the session control for the user is in the home network. A second aspect is the access-independent paradigm that the IMS is aimed to.
Currently, 3GPP multimedia networks route the IMS SIP calls through the user's home network, even if the user is roaming in a visited network. In this case, the user's SIP session is routed to an S-CSCF assigned in the user's home network so that the locations of user and S-CSCF may actually be different from a geographical point of view. Moreover, since IMS is an access-independent domain, a user assigned with a P-CSCF in the home network is not considered to be roaming from an IMS perspective even though the selected access network could be outside the home network operator premises. Thus, the S-CSCF serving a user has no way of knowing in current IMS network architecture if the user is roaming and in which country. The S-CSCF knows only a network identifier of the P-CSCF, but it cannot derive the user's geographical location thereof.
This problem gets worse when the user attaches a GPRS network as selected access network, wherein the user accesses via a Serving GPRS Service node (SGSN) to a Gateway GPRS Service node (GGSN), and the GGSN physically resides in the user's home network; or when the user attaches a WLAN network as selected access network, wherein the user accesses via a WLAN Access Gateway (WAG) to a Packet Data Gateway (PDG), and the PDG physically resides in the user's home network. This is due to the fact that both GGSN and PDG traditionally assign a P-CSCF in the same network as they are, so that the information stored in the S-CSCF, which may include a network identifier of the P-CSCF, does not always allow to determine where the user is roaming. Hence, with the presently existing architecture and mechanisms in a 3GPP IMS network, the S-CSCF handles all SIP sessions initiated for a user as local to the country of the network where the P-CSCF belongs to, even if the user is actually in a visited network in another country.
A first problem perceived by users is that a user cannot dial E.164 numbers in the local format of the country where the user is roaming because the S-CSCF may assume that the number corresponds to the country of the home network. Hence, a roaming user must dial numbers in the visited network in international format rather than in a local format. This behaviour is different from the one currently supported in GSM that allows the use of local format for numbers in the visited network. Moreover, a uniform treatment of dialled numbers for roaming users can be seen as an advantage for users wanting such uniform treatment but, on the other hand, it may be regarded as a disadvantage for users wanting to customize the dialling in accordance with user preferences.
Under the existing approach, roaming users cannot specify whether they prefer to dial numbers belonging to the home network in local format or in international format. If they decide to dial numbers belonging to the home network in local format when roaming, they must then dial numbers belonging to the visited network in international format. The S-CSCF thus originates SIP sessions as locally originated in the user's home country rather than originated in the country where the user is roaming. As a consequence, restrictions based on international calls, or on international sessions, cannot be properly applied in the S-CSCF.
Therefore, an object aiming the present invention is the provision of a mechanism whereby roaming users can specify whether they prefer to dial numbers belonging to the home network in local format or in international format from the visited network where they are roaming.
A further object aiming the present invention is the appropriate treatment of restrictions based on the type of call, and depending on both user's geographical location and preferences, for dialled number formats when the user is roaming.